Volumetric pump with replaceable reservoir assembly

ABSTRACT

An infusion pump has a removable reservoir including a piston and drive member. The drive member is an internally threaded elongated portion of the piston. A motor rotates a short drive screw, which engages the threads of the drive member to displace the piston in the reservoir. In one embodiment the piston is attached to the drive member by a severable central stem which breaks away to allow engagement with the drive screw. With the stem severed, the piston cannot be retracted, or the reservoir refilled. Other safety features are shown for locking the displaced piston in the reservoir. A digital metering system has a safety circuit to prevent dangerous failure modes of the rotation sensor and motor drive systems.

TECHNICAL FIELD

This invention relates to medical infusion pumps and more particularlypumps capable of home use by ambulatory patients and pumps having builtin controls for sustained or periodic infusions over a substantial time.

BACKGROUND ART

Much interest has arisen in recent years in the development of medicalinfusion pumps capable of relatively continuous operation and also inthe development of infusion pumps having the ability to administerprecisely controlled volumes of fluid at preselected intervals. Unlikethe case with a parenteral fluid administration apparatus, whichtypically operates by gravity flow from a relatively large reservoir offluid, medication infusion generally involves administration of smalldosages so as to achieve a prescribed optimum level of medication in thebloodstream of the patient. Due to differences in the rates ofmetabolism of, or elimination of, various medicines, the attainment ofprecise bloodstream medication levels is usually achieved by theperiodic administration of a dosage of medication, where determinationof the precise dosage and the correct interval between doses will dependon the particular medication involved as well as criteria peculiar tothe patient. Such repeated administration of medicines can be difficultto achieve because of erratic communications with, or superveningdemands, on hospital nursing staff; moreover repeatedly giving a patientinjections can be traumatic. Thus emphasis has grown on developingversatile medical infusion pumps to automatically administer a sustainedsequence or rate of infusions. Pumps of several types are known in theprior art. One species of pump involves rather massive mechanicalelements for the pump and dosage setting device so as to give the veryprecisely controlled sequence of high pressure infusions necessary forradiography. Such devices are disclosed in U.S. Pat. Nos. 3,701,345 and3,812,843, among others.

Another, general purpose, species of pump utilizes a motor or clockdrive to power a pushing mechanism, using a leadscrew or rack, formoving a plunger arrangement. Examples of such devices are my medicationinjection device disclosed in U.S. Pat. No. 3,858,581, or that of U.S.Pat. No. 4,059,110. Such devices are more compact and are reasonablywell adapted for use as bedside periodic infusion devices in a hospitalsetting. However these devices tend to be somewhat bulky and, for thisreason, are less than optimal for use on an ambulatory patient. Each ofthe prior art devices also suffers from a looseness in its mechanicaldrive components which requires close attention when setting up theinitial pumping operation to ensure that the device actually pumps afull dose during the first duty cycle of its activation. Additionallyeach of the prior art devices has a rather complex mechanical structure,introducing significant inertial and frictional factors affecting theease and cost of maintaining the units in reliable operating conditionover extended times, and making them unsuitable for battery poweredoperation.

BRIEF DESCRIPTION OF THE INVENTION

The present invention overcomes the foregoing limitations by providing asimplified mechanical structure for a pump including a reservoir, pistonand drive member, in which the drive member may be internally threadedfor a substantial length. A relatively short drive screw, which may bemotor driven, engages the drive member to propel the piston in thereservoir. By using a piston drive member in which the skirt is omittedon one side, it is possible to place the motor and drive screwsubstantially within the extended piston member body, realizing asubstantial reduction in space requirements. Additionally, such astructure requires no drive member journals, and dispenses with thecumbersome rack or leadscrew characteristic of prior art devices. Theremay result a pump structure with minimal mass, minimal frictional lossesor mechanical inefficiency, and no mechanical backlash. In a preferredembodiment the reservoir, piston, and drive member may be an assemblythat can be removed from the pump. In one preferred embodiment,interlocking detents on the piston and reservoir walls may be used toprevent withdrawal or reuse of the reservoir assembly or componentsafter use. In another preferred embodiment, a shearable stem, which maybe integrally formed with the piston face, holds the piston and drivemember together for setting up or bleeding the pump. The stem then issheared off by twisting so that the piston face floats free, preventingre-use of the reservoir assembly. The invention is well adapted for usewith battery powered drive, and microprocessor chip control programmingmeans, for dependable use with small power requirements in hospitalbedside or ambulatory outpatient settings. These and other features ofthe invention will become apparent by reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective view of a programmablemicroprocessor-controlled embodiment of the invention.

FIG. 1B shows a perspective view of an embodiment similar to that ofFIG. 1A.

FIG. 2 shows details of a disposable reservoir, in accordance with thepresent invention, with the piston and drive member withdrawn.

FIGS. 2A, 2B and 2C show various embodiments of a reservoir safetyretaining feature utilizing a lip, barb, or groove.

FIG. 3 shows a side view of the piston and drive member, showing theplacement of the motor and drivescrew within its contours.

FIG. 3A shows a cross-sectional view of the piston member and safetybarb.

FIG. 3B shows a cross-sectional view of the piston member and thebreakaway safety stem.

FIG. 4 shows a perspective view of the piston member, skirt profile anddrive engagement threads.

FIGS. 5A, 5B and 5C show different embodiments of the piston faceadapted to provide a fluid seal.

FIG. 6 shows a large volume embodiment adapted for sustained bedsideinfusion.

FIG. 7 shows a section along line 7--7 of FIG. 6, showing the motor anddrive.

FIG. 8 shows the embodiment of FIG. 6 having the retaining slide cover.

FIG. 9 shows the large volume pump of FIG. 6 including the housingadapted to house a keyboard, control circuitry and power supply.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a perspective view of a complete infusion pump accordingthe present invention, in which a housing 16, containing a power supplyand microprocessor control circuity, accomodates a motorized drive unitand a disposable piston/reservoir assembly in its upper portion. Akeyboard 7, preferably of an impermeable, e.g. membrane-type,construction is used to enter program information to set a schedule ofamounts and timing of medication doses. A display 9 is used to displaythe instrument mode, to give instructions for data entry, and to signalcertain operator correctable steps and alarm states. In the upperportion of the housing is mounted a reservoir 11 with a centrallydisposed elongated piston member 12. The reservoir 11 is secured at oneend by a gentle snap fit within opposing portions of a yoke 15 formed bythe housing body. (A single side of yoke 15 is shown in this figure.) Atthe other end of the reservoir is an axially-sliding cover portion 10 ofthe housing which overlies the end of the reservoir 11 and bears againstthe longitudinal portion of piston member 12 to align it. To minimizefriction against the piston member 12 as it slides within the reservoir11, there is provided a protuding longitudinal ridge 122 comprising thearea of contact between the piston member and the reservoir 11 andsliding cover 10.

FIG. 1B shows a perspective view of a complete infusion unit similar tothe embodiment of FIG. 1A and viewed from the opposite side. A reservoir11, which may be graduated along its surface, fits into a recess 17 inthe unit's housing 16 and is retained by the yoke 15 which varies onlyslightly from the yoke shown in FIG. 1A. The reservoir 11 issubstantially in the form of a hollow cylinder, and yoke 15 ispositioned so as to retain the reservoir in axial alignment. It has beenfound that the use of the split yoke 15 permits convenient insertion ofthe reservoir 11 into recess 17. This yoke is located to position theoutlet nipple, shown at 29 in FIG. 2, leaving space for the attachmentof a connector 181 and infusion tube 18, both of which are of aconventional design. Also shown in FIG. 1B are the piston member 12, anelongated member with a substantially circular head 13 for hermeticallydisplacing the fluid in the reservoir 11 when driven by the rotatingscrew drive 14 on the shaft of the drive motor (not visible). As shown,the housing 16 removably holds the reservoir 11 and piston member 12 asa unit. The circular piston head 13 displaces fluid in the reservoirupon axial motion of the piston member. The rearward portion of thepiston member is shaped like a longitudinal segment of cylinder as shownand is preferably internally threaded so that it may be inserted into aposition of engagement with drive screw 14. Drive screw 14 is a finelythreaded screw gear, of a diameter to mesh with the internal threads ofthe piston member 12.

In the embodiment shown, all of the mechanical elements of the pump anddrive are housed in the top portion of the housing 16 and the remainderof housing 16 serves to hold a battery or rechargeable power pack and amicroprocessor control and rate selection means for the device. A cover10, shown in phantom, slides in the direction of axis 8 so as to holdthe piston member 12 in engagement with the drive screw after thereservoir assembly is inserted in the housing. Alternatively, the cover10 may be an integral fixed part of the housing 16, and provided with aslot on the side thereof and longitudinally disposed along axis 8; theslot would be sufficient in size to permit insertion of the pistonmember 12 when the piston-reservoir assembly is first loaded into themotor assembly. After the insertion of the piston member 12 into theslot, the piston member 12 is rotated about axis 8 until the member 12assumes operating position underneath the cover, where the member 12 maybe retained by one or more nubs protruding toward axis 8 from the insideof the cover near the slot. In this manner the cover holds the piston inengagement with the drive screw.

FIG. 2 shows the reservoir 11, with its outlet nipple 29. The outletnipple may be of any conventional shape such as a taper or bayonet lock,for attachment of a sterile connector fitting 181 and infusion tube 18.When used to center the reservoir with respect to a retaining stop asshown at 15 in FIG. 1B the nipple must be of sufficient length so thatthe stop 15 does not interfere with attachment of the connector fitting.

At the other end of the reservoir is the rear edge 20 of the circularopening constituting the end of the reservoir chamber. The presentdevice is intended for use in administering infusions over extendedperiods of time thus entailing some likelihood of contamination, orincubation of bacterial contaminants. For this reason, it is necessaryto guard against misuse, particularly the possibility that reservoirunits may be removed and refilled with medication. Such refilling ispoor medical practice, and would pose a substantial risk of introducingpyrogenic, infectious or otherwise toxic contaminants into the infusionfluid.

Accordingly in one preferred embodiment of this invention, a safetyfeature has been incorporated to prevent withdrawal of the piston andrefilling of the reservoir. This safety feature includes a modificationto the inner surface contour of the reservoir adjacent to the rear edge20 of the reservoir, together with a mating modification to the distalend of the piston member. Such modifications to the reservoir assemblyare shown in cross-section in FIGS. 2A, 2B, and 2C, and may include alip 21, a barb 22, or a groove 24 located to engage the piston memberand prevent its withdrawal after use. The piston member has acorresponding barb 32 shown in FIG. 3A projecting radially outward fromits distal end which engages the mating portion of the reservoir wallfor causing the piston to be locked into the reservoir upon fullinsertion. The piston member also has a rounded shoulder 33 preferablyextending along each edge of the piston member, to prevent gripping themember which could defeat the safety lock. It has been found that thelip 21 or barb 22 of the reservoir need not protrude more than a fewthousandths of an inch above the interior face 23 of the reservoirchamber to be effective, and thus need introduce no unreasonablefrictional drag upon the piston member. Frictional drag may bealtogether eliminated by use of the groove 24 and barb 32 embodiment ofthe safety interlock.

A further preferred embodiment is shown in FIG. 3B. (FIGS. 3 and 3A arediscussed below.) In the embodiment of FIG. 3B, piston face 13 isattached to a safety stem 131, which may be integrally formed with face13, extending through aperture 121 in piston member 12, and holding face13 to member 12. Safety stem 131 is capable of transmitting force ineither direction along its length, and accordingly to set up and bleed areservoir it is only necessary to push or pull on stem 131 as for aconventional syringe. However, with stem 131 intact, it is not possibleto place the reservoir into the housing 16 or into engagement with themotor drive gear 14. Accordingly, to mount the assembly for operation,the stem is twisted, breaking off at a narrowed stress point 132, thusremoving the obstacle to motor engagement. With the stem thus removed,the face 13 is no longer attached to the piston body 12. It will advancewhen driven by the piston member, but will remain within the reservoirand will not retract when member 12 is withdrawn. Face 13 may contain apolygonal recess fitted over a corresponding polygonal protrusion ofbody 12 to prevent face 13 from turning as the stem is twisted to breakit away, thus facilitating the breaking away of the safety stem.

FIG. 3A shows a longitudinal cross section of one embodiment of thepiston member 12, in which 34 is a nub integrally formed with pistonmember 12 for retaining the front face 13. Internal screw threads 31extend the length of the inside face of the piston member back from thefront face, culminating in a rounded shoulder, 33 visible at the distaledge. On the exterior face of the piston member 12 precisely at thedistal end thereof is a slight protrusion 32 which engages thecorresponding lip 21, barb 22 or groove 24 shown at FIGS. 2A, 2B, or 2Cwhen the piston has been fully displaced into the reservoir 11, thuspreventing removal of the piston or refilling of the reservoir. In thisregard the shoulder 33 further discourages gripping of the piston, tosafeguard against removal.

Referring now to FIG. 3, there is shown in side view the piston member12 including the front face 13, which is shown somewhat occluded by thesurrounding reservoir 11. Piston face 13 may be a soft neoprene orsimilar FDA-approved compound, such as is commonly used in the plungerof disposable syringes, which is fitted over a ridged end or bulbous nubat the end of the piston member so as to be controllably moveablethereby. The precise mode of attachment is well known in the art, andaccordingly no particular detail of that structure is shown. The portionof the piston member 12 which is immediately behind the soft front face13 is substantially circular in cross-section, tapering to a long narrowbody, which is shaped substantially like a segment of a cylinder ofroughly constant thickness, threaded on the inside. The threads 31 areshown in cross-section. The precise shape of the long cylindricalsegment is not critical so long as the exterior approximately conformsto the inner contour of the reservoir and the segment is not unduly wideor narrow. For the long portion of the piston member, a width of between1/16 and 3/8 of an inch has been found to be practical, as it providesgood engagement with the drive gear 14, has adequate rigidity andcompressive strength to transmit the drive force, and is thin enough toavoid causing frictionally induced motion from the turning of the drivescrew gear 14.

Also shown in FIG. 3.is the screw gear 14 attached to the shaft of motor35. As shown in FIG. 3 the motor has its principal cross-sectionaldimension less than the diameter of the screw gear, so that it fitsentirely within the cylindrical region formed by rotating the pistonmember 12 about the common central axis of the reservoir/pistonassembly. An electromagnetic motor having an integral high-ratio gearreduction unit coaxially mounted in a common housing between the motorand drive gear has been used. This allows the motor 35 to drive thepiston member 12 directly via the drive screw gear 14 without requiringother shafts, gears, pulleys, journals or other mechanical coupling orsupporting elements in housing 16, resulting in a reduction of bothinertial load and frictional losses as compared to conventional pumps.The screw gear drive may be of any appropriate pitch with the choice ofpitch dependent on the cross-sectional area of the reservoir, thedesired delivery rates and infusion pressures, the available motor speedand torque output and the desired operational cycles.

In constructing the screw drive, the thread profile of the screw gear inrelation to the threads of the inner surface of the piston member is ofsome importance. In order to reduce backlash in the drive assembly, itis useful either to provide sharp peaks on the drive gear threads, sothat the gear bites somewhat into the trenches of the threads of thepiston member, or to provide a slight convexity or preloading in theface of the threads of either the drive gear or the piston member.

The operative metering principle involved in the control system of thepresent pump is that rotations of the drive screw 14 are directlyproportional to the linear displacement of piston, hence to the volumeof medicine delivered. The automatic control system for the present pumputilizes an internal light source which is reflected by a segmentedoptical disc connected to the drive train and detected by aphotodetector so as to generate, in a manner known in the art, pulsesrepresentative of drive screw rotation. The control system operates thepump, starting at a predetermined time, and continuing until apredetermined number of pulses have been generated.

Because the reservoir 11 holds sufficient medication for many hours ordays of therapy, extreme precautions must be taken to assure that thepumps do not have faults which might result in a continuous pumpingfailure mode. This might occur for instance if the light source failedto turn on, or burned out, so that no reflected pulses signalled thecontinued operation of the pump. To prevent the occurrence of suchdangerous events, a novel arrangement is used in the drive system of thepump. Specifically the light and drive motor are wired in series orotherwise arranged so that both necessarily receive power if and only ifthe other is receiving and drawing power. In addition, condition sensingand power enabling circuitry are provided to further assure the devicewill shut down in the event a drive-sensing component fails. Inparticular a second photodetector is provided to detect direct (asopposed to reflected) light from the internal light source; in the eventthat the second photodetector fails to detect light from the internallight source, the circuitry causes power to be removed from light-drivemotor combination. With this approach the motor is powered and medicineis delivered to the patient only if the motor, the light, and theturn-counting mechanism are all functioning. This circuit eliminates thedangers of undetected motor jamming or lamp burnout. This peculiararrangement of the operating components with the logic and conditionsensors necessarily assures that none of the permutations of componentfailure can result in a dangerous operating condition, but merely inshut-down of the device.

The foregoing discussion is not restricted to a rotation sensing systemusing reflected light, and it would be obvious to one skilled in the artto construct a corresponding safety system for use in a rotation sensoremploying a slotted disc and a transmitted beam of light. Accordinglythe terms "reflected" or "reflecting" shall include "transmitted" or"transmitting" when used herein and in the claims. The precise detailsof the signal generation and detection also may be put into practiceusing a variety of components in a manner well known in the art.

Turning now to FIG. 4, there is shown the piston member generally at 12having internal threads 31 disposed along the length of the pistonskirt, the portion of piston member 12 which extends laterally outwardfrom piston head 13 and receives rotating screw gear 14. By engagementwith the short rotating screw gear 14 the threads are operative to drivethe piston a distance equal to the length of the threads 31. Because thepiston skirt must slide laterally into engagement with the drive screw,the skirt comprises not much more, and preferably less, than asemicircumferential portion of the cylindrical surface. In fact, sincethe piston skirt is used to transmit a linearly-directed drive force,shear forces are negligible; and because the piston head 13 is joined tothe skirt portion along an arcuate edge, the skirt may safely be quitenarrow.

Such a drive member 12, having a narrow skirt, is shown in FIGS. 5A, 5Band 5C, each of which shows an alternative embodiment of the pistonface. In FIG. 5A there is shown a piston member having a head 13 inwhich a circumferential groove holds a sealing ring 54. In FIG. 5B avariation of the piston head with sealing ring 54 is shown, furtherhaving a flared cup 55. The flared cup provides a strong seal againstfluid leakage in the direction opposing the flare. In FIG. 5C is shown apiston member having an integrally-formed head with a frontal flared cup55, are a spaced-apart rear portion 56, which, because of its thinness,may be of a large diameter and operate as a bidirectional seal andscraper The piston skirt in any of these embodiments preferably has oneor more longitudinal ridges, 122 in FIG. 2, arrayed along its exteriorsurface, to minimize the area of contact with the reservoir wall and todiminish frictional drag.

Turning now to FIG. 6, there is shown an embodiment of the presentinvention usable as a large volume or macro-pump which may be suspendedfor bedside use in a vertical orientation in a manner similar to aconventional gravity infusion reservoir. The reservoir and driveassembly of the macro-pump are scaled-up versions of the device of FIG.1, having a housing 66 which accepts a reservoir 61 having a pistondrive member 62. The reservoir is removably attached to the housing asby lip 68 of the reservoir and circumferential groove 69 in the housing,or by a breech lock or similar arrangement. The piston member 62includes a piston face 63, which may be further sealed using an O-ringor any of the structural variations shown in FIGS. 5A, B or C.

In FIG. 7 is shown a cutaway view along line 7--7 of the embodiment ofFIG. 6, in which a motor 35 and drive screw 14 are shown driveablyengaging internal threads of piston member 62. FIG. 8 further shows aslot in housing 66 for holding piston drive member 62 in alignment.After insertion of the reservoir assembly into the housing a slidingplate 80 may be moved along axis 88 to close the slot and hold thepiston drive member 62 in engagement with screw gear 14. Alternate meansfor biasing the drive assembly are possible. Furthermore, while FIG. 7shows a drive shaft journalled in the housing, such housing journals arenot necessary, and the drive screw assembly may be simply mounted on themotor shaft, and the motor attached to the housing.

Because of the rigidity of the reservoir and the precise volumetricrelationship between drive rotations and piston displacement themacro-pump embodiment is readily adaptable to microprocessor drivecontrol and information display in a manner to yield extremely precisedosage control. This rigid piston and reservoir configuration eliminatesthe air space characteristic of bottle reservoirs (which consequentlyrequire venting, drip chambers, and other equipment) and the undefinedvolume characteristic of bag reservoirs (which do not permit readyindication delivered fluid volume), achieving in a single apparatus theprecision normally associated with separate, expensive, dosing pumps.The housing 66 may house both a microprocessor controller and a batterypower supply.

FIG. 9 shows such a device, in which the housing 16 includes a keyboard7 on one face, comprising a small number of programming keys. The panelmay incorporate a display similar to 9 of FIG. 1, of a kind known in theart, on which may appear messages to guide an attendant in entering theprogram data related to reservoir volume, total dose and timing. Such adisplay may also display relevant information such as the totaldelivered dose, or time interval since last infusion, all of which maybe conveniently coded, stored and retrieved starting with the basicoperating pulse data of the pump drive and the coded program data of themicroprocessor control, by techniques known in the art.

It should be noted that although the foregoing discussion speaks of thefabrication of one embodiment using a motor and reducing gear, it is byno means necessary that the motor be an electromagnetic rotary motorcapable of continuous operation. Indeed, a stepping motor, or even asolenoid or piezo ratchet driven motor would be equally serviceable, aslong as the device were capable of having its total angular rotationprecisely controlled or monitored by the control means. Whereas acontinuous duty rotary motor may prove best for conveniently infusing aninitial slug of medication, a stepping motor or ratchet drive may provethe most efficient or reliable for sustained infusion of maintenancedoses at very low rates. In that event, a solenoid-actuated ratchet gearand corresponding digital control signalling means may be provided inthe space shown for the motor. Furthermore, while the piston member hasbeen shown as having a long thin body for simplicity, the relevantstructural limitation is that the piston member comprise not much morethan about 180° of arc of a cylindrical surface so that it may beconveniently placed in engagement around the drive gear. Thus aplurality of suitably spaced elements with internal threads in registry,or a wider piston member could be employed instead of the single thinpiston body shown.

Accordingly, while the invention has been described with reference tospecific embodiments thereof, it is to be understood that the inventionmay be embodied in other forms without departing from the spirit andscope of the invention as defined by the following claims.

What is claimed is:
 1. A volumetric pump, comprising:reservoir means forproviding a reservoir of variable volume, including a piston locatedwithin the reservoir means which when axially displaced causes a changein reservoir volume; drive means in mechanical connection with thepiston, for displacing the piston when such means is engaged against arotating member having a component of angular velocity in the directionof piston displacement, such drive means having an interior drivesurface that is substantially a longitudinal section of a surface ofrevolution about a first axis substantially parallel to the direction ofpiston displacement, such interior drive surface being threaded for alength at least as great as the desired total piston displacement; arotating, axially non-displaceable drive screw which is short incomparison to the displacement of the piston and has an axis or rotationapproximately coincident with the first axis, in engagement with theinterior drive surface, for displacing the piston; wherein the reservoirmeans and drive means form an assembly that is removably engageableagainst the drive screw; and sever-connection means, for temporarilyconnecting the piston to the drive means, for movement of the drivemeans when so connected to the piston both to increase and to decreasethe volume of the reservoir means, in such a way that thesever-connection means must be severed to permit placing of the drivemeans in engagement with the drive screw, and so that following suchseverence the piston is no longer connected to the drive means and canthen be moved only to decrease the volume of the reservoir means.
 2. Apump according to claim 1, wherein the sever-connection means includes astem, one end of which severably connects the piston and the drive meansand the other end of which serves as a handle to manipulate displacementof the piston before the stem is severed.
 3. A reservoir assembly forremovable engagement with a motor drive, comprising:a reservoir; apiston having (i) a fluid contacting head portion located within thereservoir and (ii) a drive body for engagement with a motor drive andhaving an aperture therein; a severable stem, removably attaching thefluid contacting head portion to the drive body, protruding from thefluid contacting head portion through the aperture in the drive bodysufficiently far to prevent the drive body from being engaged with themotr drive without the stem first having been severed, so that when thepiston is placed in engagement with the motor drive the head portion isdetached from the body and retraction of the head portion and refillingof the reservoir are prevented.
 4. A reservoir assembly according toclaim 3, wherein the stem extends along a central longitudinal pistonaxis and blocks the motor drive from engaging an internal surface of thepiston.
 5. A reservoir assembly according to claim 3, wherein the stemis of sufficient strength to transmit push and pull forces for fillingand bleeding the assembly, but has a weak portion which may be severedby twisting, so as to detach the head portion from the body.
 6. Areservoir assembly according to claim 5, wherein the stem is integrallyformed with the piston head portion.
 7. A reservoir assembly accordingto claim 4, wherein the drive body has affixed thereto a pistonfollower, threaded along its interior for a length greater than thedesired total piston displacement.